Patent Application
20090045692
The technology disclosed herein relates generally to the repair of rotary machines and, more specifically, to wedges used for the retention of conductor (or stator) bars in the stator core slots of dynamoelectric machines.
Large dynamoelectric machines such as electrical generators employ a laminated stator core for transmitting induced voltages to the generator terminals through stator conductor bars. The cores are usually made by assembling already-slotted punchings or laminations in an annular housing for later containing the generator rotor. The slotted punchings, when assembled, define axially-extending radial slots which terminate at the radially inner-circumference of the stator annulus. The stator bars, or conductors, are laid in the radial slots and a wedging system is used to hold the bars in place against electromagnetic forces present when the machine is operating. If the wedging system is not effective, conductor insulation may be damaged in the ensuing vibration, ultimately leading to a forced outage of the generator.
Electromagnetic fields in the generator induce forces on stators bars during normal operation or short circuit conditions that require wedges to support and hold the bars within the stator slots.
Currently fiberglass laminate material (such as, for example, National Electrical Manufacturers Association (NEMA) G11) is used in making the wedges, and while G11 provides good mechanical strength, it is abrasive to the stator laminations.
Cotton phenolic material has also been used as a wedge material which is non-abrasive to the core but has lower thermal and mechanical capability versus fiberglass laminates such as G11. The reduced mechanical strength and thermal capability of cotton phenolic limits the application of wedges made using this material.
Other solutions such as low friction coatings have not proven completely successful, primarily because they are insufficiently abrasion resistant.
In U.S. Pat. No. 4,200,818, there is disclosed a stator wedge partially covered with a non-woven felt made of Kevlar®, and U.S. Pat. No. 4,607,183 discloses a wedge with an abrasion resistant layer.
There remains a need for wedges exhibiting the properties of fiberglass laminates such as G11 but that have nonabrasive surface for use in dynamoelectric machines.
In one aspect, the present invention relates to a slot wedge for a generator stator comprising a wedge body having top and bottom surfaces and a pair of oppositely inclined side surfaces, wherein at least said oppositely inclined surfaces are covered with a woven aramid fabric. The woven aramid fabric provides a non-abrasive interface between the fiberglass wedge body and the stator core laminations.
In another aspect, the invention relates to a method of making a slot wedge for a generator stator comprising: (a) providing a wedge shaped body having top and bottom surfaces connected by oppositely inclined side surfaces; and (b) covering at least the oppositely inclined side surfaces with a woven aramid fabric. The woven aramid fabric provides a nonabrasive interface between the fiberglass wedge body and the stator core laminations.
The stator wedge technology disclosed herein will now be described in detail in connection with the below identified drawings.
In conjunction with the foregoing, a filler strip 24 may extend axially (longitudinally) along the slot radially inward of bar 22. A number of dovetail wedges 26 are introduced into the slot 14 (and spaced apart along the axial length of the slot 14) so as to bear radially against the insulating filler strip 24. The dovetail wedges are formed with oppositely-facing inclined surfaces 28 which engage inclined surfaces of the dovetail slot 16 to facilitate the assembly of the stator bar wedging system. The material of the dovetail wedges 26 is preferably of high-strength insulating material which can be cut or molded to the desired wedge shapes. The wedges are thus preferably formed of a molded resinous compound employing a suitable filler to add strength, or in the alternative, are formed of any suitable commercially-obtainable cotton phenolic materials such as Textolite® (a registered trademark of the General Electric Company). In some designs cotton phenolic wedge by itself lacks the required mechanical strength for thinner wedge configurations.
With reference to
The covered wedge as described above may be manufactured by, for example, any of the following methods.
In a first exemplary process, liquefied G11 resin is poured into a mold cavity containing a woven glass roll the length of the wedge 30. The woven aramid fabric 32 is placed over the resin and the assembly is pressed into final shape with heat and pressure.
In a second exemplary process, liquefied G11 resin with woven glass fibers and the woven aramid fabric is pulled (pultrusion) or pushed (extrusion) through a die that produces the desired wedge shape.
In a third process, the G11 resin is shaped by either of the above processes, and the woven aramid fabric is thereafter glued to the wedge.
Other known processes may be equally suitable for forming the woven aramid fabric-covered wedge as described, but it is important that the aramid fabric be bonded to the fiberglass laminate to prevent delamination during use.
While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.
